Prior art remote display controllers provided control from a control location to a remote display or sign by means of direct electrical connection or radio wave communication. In such systems, a signal was transmitted from the remote location to the display. One of several predetermined messages stored at the display was selected and displayed. In some cases, a provision was made to acknowledge receipt of the signal. The display, however, did not provide to the control location a positive response indicating that the correct message had been selected, or that the message had been properly displayed.
The prior art systems also offered no means for providing information to the control location relating to the status of the display itself, as distinguished from the displayed message.